286 METALLURGY 0.¥ IRON AND STEEL.
approximately the same size, the readings are relative, and being relative may be considered to be correct. Now is this true under conditions radically different ? If a small piece of steel is placed in a muffle and heated, the muffle having been at a high temperature before the introduction of the piece, it will be found even while the piece is black or very dark red, say not over 650° C., that the needle of a Le Chatelier pyrometer, the couple of which is in contact with the steel, will indicate a temperature some thirty degrees higher. This is probably due to the fact that while it takes some time for the mass of steel to absorb the heat from the muffle, the fine wires of the couple arrive at the high temperature in perhaps twenty or thirty seconds. Of course, the juncture, being in contact with the cooler steel, is considerably cooler than the furnace, but nevertheless it is some degrees higher than the piece, and this higher temperature is the one which sets up the difference of potential which affects the galvanometer.
This is undoubtedly the case in still greater measure with larger furnaces and larger masses, and if it is desired to compare a small piece with a large one the temperature of treatment must be the same. There is one way of arriving at this with certainty, and this is in accordance with what Howe describes as the condition of invisibility. He sets forth that a certain color is indicative of a certain temperature, whatever the material, and proves it by stating that if pieces of several different kinds of metals be placed in a furnace and heated carefully and slowly, and held till it is certain that they are heated equally through and through, on looking into the furnace nothing can be seen but the walls of the furnace. The pieces are invisible. He then shows that since the only light is that given off by the heated surfaces themselves and since if there were even the slightest difference in color, the edges of the pieces could be seen, the whole furnace and contents must be the same color and this he calls "invisibility.-"
Now if a large piece of metal is heated until the wires of the couple cannot be seen in contact with the piece, and if this heating be continued until the piece shows an uniform color all over its surface, and until it has been heated throughout to this color, an absolute reading is obtained—at least absolute within the limits of error of the galvanometer. In this connection it should be stated that the Le Chatelier pyrometer is the best practical method of taking readings, of high temperatures. That a piece